This invention relates generally to centrifugal compressors and, more particularly, to an improved lubrication method and apparatus therefor.
Hermetic centrifugal refrigeration compressors generally use an electric motor to drive the impeller through a geared up transmission. In such a compressor, the transmission is typically vented to a source of low pressure within the chiller to minimize the outward migration of oil through the shaft seals. It has been recognized that during this venting process, in addition to the refrigerant gas passing out of the transmission, some of the oil in the form of droplets or mist tends to become entrained within the refrigerant gas so as to also pass out of the transmission. This has not been a particular problem with conventional refrigerants of moderate pressure and densities.
With the more recent use of higher pressure and higher density refrigerants, such as R-22, the above described problem of oil carry-over has become more significant. That is, because of the higher pressure refrigerant, it is necessary to operate with larger gears operating at higher speeds. This, in turn, increases the turbulence which tends to cause large volumes of lubricant to be drawn up from the integral sump and enter into the compression cycle by way of the low pressure vent opening. Further, the larger pressure differentials tend to promote higher vent gas flow rates and therefore increased carry-over.
In addition to the refrigerant higher pressures, the higher densities also tend to exasperate the problem. That is, the increased densities tend to keep the oil droplets in suspension longer and makes separation more difficult. In addition, the higher densities also increase mechanical losses from centrifugal type oil separation mechanisms.
Considering now the result of oil carry-over that occurs when the oil that is drawn up from the sump by turbulence is vented to the compressor inlet, it passes through the compressor and is discharged into the condenser, where it tends to coat the heat exchanger surface to thereby decrease the efficiency thereof. Some of the oil is then passed on to the cooler where the same phenomenon occurs. Thus, it will be recognized that high oil carry-over rates tend to result in reduced heat exchanger performance. Moreover, as the oil supply in the sump is diminished because of this phenomenon, there may no longer be a sufficient amount of oil to ensure that all of the moving parts that require lubrication are in fact receiving adequate supplies of oil.
The oil carryover problem has been addressed in two different ways. First, the most common approach is to use a mesh type oil separator in the vent line to cause oil droplets to coalesce and drain back into the transmission. A second method uses a series of hollow rotating spokes to centrifuge out the unwanted oil mist component of the vent flow. Neither of these methods, by themselves or in combination, are found to be sufficient for containing oil in a centrifugal compressor using high pressure, high density refrigerant such as R-22.
It is therefore an object of the present invention to provide an improved oil containment system for a centrifugal compressor.
Another object of the present invention is the provision for limiting oil carry-over in a centrifugal compressor.
Yet another object of the present invention is the provision in a centrifugal compressor using high pressure, high density refrigerant, for limiting the high oil carry-over rates that would otherwise occur.
Still another object of the present invention is the provision for a transmission oil containment system that is economical to manufacture and effective in use.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.